1. Field of the Invention
The present invention relates to a map display apparatus, a map display method, and a map display program product, which are used for a navigation device and the like.
2. Description of the Related Art
A navigation device which is generally mounted on a vehicle or the like grasps a position of the vehicle using a GPS (Global Positioning System), and guides a route to a destination which is instructed by a user. A typical navigation device performs a displaying of the vehicle position, the guiding of the route, and the like using a map display apparatus mounted in the navigation device. When a request for the map displaying is issued for a region where the navigation device or the like is positioned, the map display apparatus reads out map data on the requested region from a recording unit in which the map data is recorded, generates map image data, and then displays the map image data in a display unit.
A general map data used in the map display apparatus includes detailed city map data in which roads, traffic signals, buildings, and the like are recorded in detail as data, and simply digitized road network information by making intersections as nodes and the straight lines (links) connecting the nodes as roads. The map display apparatus performs the map displaying while appropriately switching a map displaying using the detailed city map data and another map displaying using the road network information. For example, when the detailed city map data is present in a target region of the map displaying, the map display apparatus basically performs the map displaying using the detailed city map data. In this case, however, a display scale of the requested map displaying is a broad display mode of which the scale exceeds an allowable range of the map displaying of the detailed city map data, the map displaying is performed using the road network information instead of the detailed city map data. In addition, since the detailed city map data is provided by only a part of an area of high population density zone but not covers all areas, when the map displaying is applied to an area having no detailed city map data, the map display apparatus performs the map displaying using the road network information. The traffic signal in the map displaying using the road network information is displayed depending on whether a node representing the intersection has attribute information of traffic signal available. In other words, when the node representing the intersection has the attribute information of traffic signal available, a traffic signal icon is displayed at the position of the node.
In this case, however, for the improvement of accuracy in map matching between the subject vehicle position and roads in the road network information, wide roads as illustrated in FIG. 6A are not digitized using one link; for example, as illustrated in FIG. 6B, a plurality (two in FIG. 6B) of links L, each of which is separated into an upper lane and a lower lane, may be digitized.
However, when the above described digitization is being performed, for example, the intersection between the roads digitized into two links L is constituted by four nodes denoted by “” in FIG. 6B. When a traffic signal is present in such an intersection, each of the four nodes has the attribute information of traffic signal available, and four traffic signal icons are displayed in one intersection. For this reason, a plurality of traffic signal icons may be densely displayed or may be displayed in an overlapping manner depending on the display scale of the map displaying, and thus it may be difficult to grasp the map displaying. In addition, for example, a user may misunderstand the intersection as a complex intersection, and thus it may be difficult to grasp the traffic signal positions and the road conditions.
In this regard, Japanese Laid-open Patent Publication No. 2001-125702 discloses an invention relating to a display control method in which a representative icon is displayed instead of individual icons when a plurality of icons are displayed in an overlapping manner.